[1] Mihajlo Pupin Institute, Volgina 15, 11000 Belgrade, Yugoslavia
[2] Department of Engineering, Univ. of Warwick, Mfg. Group, Coventry CV4 7AL, United Kingdom
[3] Fac. of Traffic and Transp. Eng., Univ. of Belgrade, Vojvode Stepe 305, 11000 Belgrade, Yugoslavia